1. Field of the Invention
The present invention relates to an apparatus and a method for determining the validity of reflected pulse signals such as sonar and radar signals and in particular to a device and method which use binary threshold detection in combination with a third threshold to distinguish between strong and weak signal returns and to adapt the system or method detection criteria accordingly.
2. DESCRIPTION OF THE PRIOR ART
Various devices and methods have been devised to increase the ability of radar and sonar systems to accurately detect a valid reflected pulse signal. These include such devices as comb filters, statistical filters, threshold circuits and the like. In each instance the objective is to distinguish a valid reflected signal from unwanted or invalid signals including background noise, interference from other friendly systems, and jamming signals, while simultaneously insuring that weak but valid signals are not rejected. in a real world environment, valid signals, noise, and interferance signals will range from weak to strong.
Various devices and/or methods have been devised to increase detection of weak valid signals while simultaneously increasing rejection of invalid signals from sources such as background noise, clutter, and jamming devices. One such device is the binary detector or double thresholder. In this device, a received signal is initially envelope detected. The pulse envelopes are first tested against an amplitude threshold. If the signal amplitudes are less than the predetermined amplitude threshold value they are rejected. If the signal amplitudes occurring at a sampling instant are above the threshold value they are counted and the number of pulse envelopes is compared against a second threshold which is a predetermined number of pulse envelopes or zero crossings that occur out of a maximum number of such zero crossings that can occur. If the number of pulse envelopes equals or exceeds the second threshold, a valid detection is established and the signal is output to a tracker or other device. This method of detection increases rejection of randomly arriving impulse interference caused by other nearby systems or from jamming devices inasmuch as the time coincidence of such interference signals with a valid incoherent pulse train reflection is small. A detailed discussion of such a detection device and method is presented in the text "Radar Detection" by J. V. DiFranco and W. L. Rubin and published by Prentice Hall.
In such prior art devices and methods, however, the dichotomy of increasing system sensitivity to ensure detection of all valid signals, including weak signals, with an attendant increase in detection of false or invalid signals versus increasing system selectivity to reduce false detection with an attendant loss of sensitivity and detection of weak signals, persists. There therefor exists a need to provide a system and method for the detection of reflected pulse signals which provides increased sensitivity while simultaneously preserving or increasing system selectivity.